It sounds like you know the life codes pretty good, Maybe the girl will move with you to work at Famous Corp in 2019? It might take a ring to persuade her. I lived in Taiwan 3 years as a teenager, I loved the country, so I know a little about living there. At 27 there is still plenty of time to get everything ready for what will be your future. So as long as there is a plan it will work out for you. I think that maybe Famous Corp doesn't want you to tell the Boss because it is actually their way of saving face for "poaching" you or they don't wish to get entangled with your present employer. It is actually pretty standard procedure, whats different is that you are actual friends with your boss.
The main problem seems to be ,should I tell my boss now... because he is your friend the only answer is yes. anything less will likely haunt you for years. Famous Company does not need to know who you told about their offer, so do not tell them.
Tell the boss now, then go get the offer, with a solid offer in hand ask the girl what "we" should do, then ask boss what he thinks. I'm sure that whatever advice they give will be better than anything else you will find.